I'm so very sorry for your loss of Mattie. I agree, the pain, loss, and emptiness you feel is unbearable and there are no words really to describe how you feel.
There are too many of us who know what you're going through, I am one of them. I lost my boy almost 2 years ago at the age of 15.5. We were devastated and miss him everyday.
Time will help ease your pain, each day will get a littler easier, although you will always miss them. I hope the day will come where you will be able to find peace in knowing that Mattie is at the Rainbow Bridge with so many of our Goldens. I find comfort in knowing that my boy is no longer in pain, he is running free, enjoying life like he once did as a pup, and waiting for me to join him.
My heart goes out to you.
Godspeed sweet Mattie